Hi I need to power up an external device which uses 3V batteries with arduino, based on a GPRS call. I already have the GPRS shield, and this is working fine, I can switch on a pin whenever I want. The problem is about the circuit needed.

I tried to use the 5V ouput into a transistor controlled by the base, a 27 Ohm resistor and then connect it to my device, but this doesn't work. So I tried to use my out pin (which gives me around 4.5V) and use the same resistance, and then this goes to my device's + pin, but this didn't work either.

I am pretty newbie on electronic, can anyone give me an advice to do that?

A 27R on the base is far too small make it a 2K7 at least. Are you trying to control the power to this device or send a signal to it?If a signal then :-Then on the collector you need a pull up resistor, say 1K to the 3V line.

A 27R on the base is far too small make it a 2K7 at least. Are you trying to control the power to this device or send a signal to it?If a signal then :-Then on the collector you need a pull up resistor, say 1K to the 3V line.

Yes this is not the way to do anything. You can not use resistors to reduce the voltage.You need to get a 3V voltage that is capable of powering your device. How much current does your device take?Then you need to switch that with a transistor.Arduino output -- 2K7 --- transistor baseTransistor emitter to grounddevice ground to transistor collectordevice power to your 3V supply +ve3V supply ground to arduino ground.

Perhaps th best way would be a low dropout 3.3v regulator, or second maybe cheaper just 2 plain diodes to drop 1.4 volts to 3.6, maybe a germanium diode to drop aanother.3v, that will work the only problem is the voltage dropped changes slightly with current being drawn